The moment Microsoft announced it would lock other browsers out of being installed on Windows RT, we all knew regulatory bodies the world over were wringing their hands. Today, this has been confirmed: in the wake of an investigation into Microsoft not complying with the existing antitrust rulings regarding browser choice, the EU has also announced it's investigating Windows 8 x86 and Windows 8 RT (ARM).

and I can't see how the situation is the same: even though iOS dominates the tablet, it is far from dominating the web

Well, that's easy to point out - codecs. Apple dominates the web with allowing usage of only MP3 and H.264 on their devices for web audio/video, and banning browsers that could allow using open codecs (Vorbis, Theora, VP8 and etc.). This way they indirectly push Web developers to use these closed codecs if they want to target Apple mobile devices (which are a big part of the market and can hardly be ignored).

Essentially Web publishers are forced to encode their content twice - in open codecs for normal browsers, and in closed codecs for mobile Safari (same story will be with Windows RT it seems). Encoding in closed codecs requires licensing if publishing has commercial purpose. Plus doubling hosting space costs money and less efficient closed codecs waste more energy, being bad for the environment. So in essence Apple does dominate the web with their ban on alternative browsers for iOS.

IE on mobile MS devices has the same issue. On the desktop, IE can be remedied with installing plugins to support open codecs, while on mobile devices with their crazy restrictions this is impossible.